In Chrome 86, a problem was introduced to the way the area of the impact fraction is calculated for layout shifts of nodes containing positioned content that overflows to the other side of the viewport. In Chrome 87, the problem is fixed by using visual rects to calculate the impact area instead of layout overflow rects (source code for the change here).
A bug fix was made to how the overflow of shifted elements inside a container that is styled
contain:paint is calculated, reducing the computed size of the shift area for these nodes. Source code for this change.
Sites whose CLS scores were negatively impacted by the regression in impact fraction calculation in Chrome 86 should return to accurate layout shift scores in Chrome 87. Developers can test in Chrome 87.0.4280.51 and later to see the impact of this change.
Sites with contain:paint styling on elements may see an improvement in CLS scores in Chrome 87.
Most users will be updated to Chrome 87 the week of November 17, 2020.